The natural adhesive capability of the gecko foot allows the animal to adhere to surfaces of many types under most conditions. The adhesive capability is provided by numerous hair-type extensions, called setae, on the feet of the gecko. Gecko setae include stalks having diameters in the range of 5 micrometers. At the distal end, each stalk branches out into nano-sized spatulae, with roughly 100 to 1000 spatulae on each stalk, each of which is about 0.2 micrometers in length. Adhesion between the spatulae and a contacting surface is obtained due to van der Waals forces. The attractive forces between a single spatula and a surface can be on the order of 100 nanoNewtons (nN). The setae can be readily separated from the surface by the animal curling its toes off of the surface from the tips inward. This peeling action alters the angle of incidence between millions of individual spatulae and the surface, reducing the van der Waals forces and allowing the animal to move across the surface.
Multiple methods have been proposed for the formation of synthetic gecko setae. For instance, Full, et al. (U.S. Patent Application Publication No. 2004/0005454) describe several formation methods, including an etching process in which a recess is etched in a semiconductor substrate, nitride and oxide layers are deposited, the surface is patterned and etched, and the underlying substrate is etched, causing the oxide and nitride layers to curl away from the substrate and form a shaft structure. Another method of Full, et al. includes utilizing an excitation source to apply energy to a sensitive material. Upon application of the energy, the sensitive material exhibits a change in volume and the altered volume is selectively etched away, forming extending tubes. Other methods of Full, et al. include polymer etching, molding by use of a narrow glass tube, lithography methods utilizing electrostatic attraction to ‘grow’ a layer of protrusions on a substrate, and utilization of a nano-imprinting roller.
Fearing, et al. (U.S. Patent Application Publication No. 2003/0208888) describes a method for forming a microstructure including a stalk that supports a protrusion at an oblique angle relative to a supporting surface. The described method includes molding a structure having the desired shape, the structure is then removed to form a template, and the template is molded with a polymer. Other formation techniques have been described by Sitti, et al. (Nanomolding Based Fabrication of Synthetic Gecko Foot-Hairs, Proceedings of the 2nd IEEE Conference on Nanotechnology, 2002) including a first method in which an AFM tip is used to form dimples on a deformable surface, and a second method using a nano-pore membrane as a template.
Such 3-D molding processes have proven problematic due to the very high aspect ratio of the structures that are formed and must be removed from the mold. Formation of hierarchical structures that include both micro- and nano-sized portions has proven particularly challenging and typically requires multiple molding steps.
Substrates including arrays of synthetic gecko setae including hierarchical micro- and nano-sized portions and methods for forming such substrates would be useful in the art, for instance in development of fasteners for a variety of articles.